1. Field of the Invention
The present invention relates to a WDM optical transmission system which transmits a wavelength division multiplexed (WDM) light while amplifying it, and an optical amplifying apparatus, and in particular, to a WDM optical transmission system and an optical amplifying apparatus, in which the optical amplification is controlled so that a signal output level is held constant.
2. Description of the Related Art
In the case where an optical amplifier is used in a WDM optical transmission system, as methods of controlling an optical amplification operation of the optical amplifier, there have been known an automatic gain control (AGC) and an automatic level control (ALC). The automatic gain control is a method of setting a gain according to a transmission path loss at the time of device setting-up, and thereafter, continuing to hold the set gain. Further, the automatic level control is a method of setting a gain according to a transmission path loss at the time of device setting-up, and thereafter, variably controlling the set gain based on wavelength numbers information on a WDM signal light being transmitted, so that the total signal output level of the optical amplifier is held constant.
However, in the above automatic gain control, there is a problem in that an output signal level of the optical amplifier is varied due to a change in the transmission path loss. For example, a loss of a transmission path fiber (transmission path loss) is changed due to the temperature. Further, there is also a possibility that stress, such as bending or the like, is subjected to the transmission path fiber for some reason, and therefore, the transmission path fiber loss is changed. If the automatic gain control is performed under such a condition, there is a possibility that the output signal level of the optical amplifier is varied according to the change in the transmission path loss, to be deviated from an optimum signal level, from a standpoint of signal reception sensitivity.
Further, in the above automatic level control, there is a problem in that the signal level variation occurs since it requires substantial time to obtain the wavelength numbers information on the WDM signal light. Namely, in order to realize the automatic level control, it is necessary to obtain information on “the number of wavelengths” of the WDM signal light which is currently being amplified by the optical amplifier. This is because, in the automatic level control, the gain of the optical amplifier is variably controlled, with the total signal level obtained by multiplying the signal level per one wavelength by the number of wavelengths as a target value. In order to detect the number of wavelengths of the WDM signal light in which a plurality of optical signals of different wavelengths are bundled, a detecting section that measures a level per one wavelength to detect the presence or absence of an optical signal is present needs to be disposed.
In the meantime, as shown in FIG. 4 for example, the number of wavelengths of the WDM signal light which is fed to an optical amplifier on the WDM optical transmission system is also arbitrarily changed during communication service operation, by applying an optical transmission apparatus, for example an optical add drop multiplexer (OADM), which is provided with a multiplexer/demultiplexer or the like on the upstream side of this optical amplifier. Accordingly, in the optical amplifier for WDM light transmission, in order to cope with the change in the number of wavelengths of the WDM signal light, a high-speed automatic gain control is usually performed.
The method in which only this high-speed automatic gain control is performed corresponds to the above described “automatic gain control”, and “automatic level control” is for performing the automatic level control using the wavelength numbers information, in addition to this high-speed automatic gain control.
As described in the above, in the automatic level control using the wavelength numbers information, since the high-speed automatic gain control is performed simultaneously with the automatic level control, there is a relation, as typically shown in FIG. 5 for example, between a control speed for holding the signal level constant and a detecting time of the number of wavelengths to be used for the automatic level control. Namely, in the case where the detecting time is longer than the control speed, although the number of wavelengths is actually changed during a period of time until the variation of the number of wavelengths is detected after the number of wavelengths of the WDM signal light is varied at the time t0 and the total input power is changed, an erroneous automatic level control is performed so that the total output power approaches a level corresponding to the number of wavelengths before changed (refer to the broken line in the lower stage of FIG. 5). Then, at the time t1 where the variation of the number of wavelengths is detected in the detecting section and information thereof is transmitted to the optical amplifier, such an erroneous automatic level control which has been performed until that moment is suspended and the automatic level control for achieving the total output power according to the post-varied number of wavelengths is performed. Therefore, the level variation ΔP is caused in the total output power. In other words, there is the necessity to set the control speed for the automatic level control to be lower, so that the level variation ΔP reaches minimum.
From the above relation, there is no problem for example in the case where the automatic level control of low control speed for absorbing the low-speed variation, such as the temperature variation of the transmission path loss, is performed. However, in the case where stress, such as bending or the like, is subjected to the transmission path fiber, when the signal level of output light is led-in in a moment of time (for example, in millisecond (ms) order) by the automatic level control, there is the necessity to speed up the detecting time of the number of wavelengths.
Namely, in order to cope with the instantaneous signal level variation, such as the bending or the like in the transmission path fiber, it becomes necessary to perform the high-speed automatic level control, and also, in order to reduce ΔP at the variation time of the number of wavelengths, it is necessary to transmit the wavelength numbers information at a high speed, so that the automatic level control is suspended and the automatic gain control is performed.
However, in the detection of the number of wavelengths, due to constraints and the like on a detecting device, there may be the problematic case where the speeding-up of detecting time is difficult. Consequently, in practice, it is difficult to approach ΔP to 0, and accordingly, the detecting device is designed such that the level variation of certain degree is allowed at the variation time of the number of wavelengths.
To cope with such a problem, as the automatic level control which does not need the speeding-up in the detection of the number of wavelengths, there has been proposed a method in which a loss (span loss) in a transmission span to a former-staged adjacent repeating device is always monitored, and the output signal level of the optical amplifier is controlled to be constant according to a monitoring result of the transmission pass loss (to be referred hereunder as “automatic level control by the span loss monitoring) (refer to Japanese Unexamined Patent Publication No. 11-261490).
FIG. 6 shows a configuration example of an essential part of a WDM optical transmission system applied with the automatic level control by the span loss monitoring. In this configuration example, a signal output level of an optical amplifier 111 which is disposed in an optical amplifying unit 110 positioned on the upstream side of the system is detected by an output monitor 112, and signal output level information thereof is transmitted from a control circuit 113 to a supervisory control light (optical supervisory channel (OSC)) transmitter 114. Then, a supervisory control light containing the signal output level information is generated by the OSC transmitter 114, and is multiplexed with a WDM signal light by a multiplexer 115 to be output to a transmission path fiber 101, and then, is transmitted over the transmission path fiber 101 toward an optical amplifying unit 130 on the downstream side of the system. In the optical amplifying unit 130, the light transmitted over the transmission path fiber 101 is demultiplexed by a demultiplexer 132 into the WDM signal light and the supervisory control light, so that the signal output level information of the optical amplifying unit 110, which is contained in the supervisory control light, is detected by an OSC receiver 134 and also a signal input level of the WDM signal light to be fed to an optical amplifier 131 is detected by an input monitor 133, and then, detection results in the OSC receiver 134 and the input monitor 133 are transmitted respectively to a control circuit 135. Then, in the control circuit 135, a span loss in the transmission path fiber 101 is calculated using the signal output level on the upstream side and the signal input level on the downstream side, and a gain of the optical amplifier 131 is set based on the span loss, so that the automatic level control for the WDM signal light output from the optical amplifier 131 is performed.
In the automatic level control by the span loss monitoring as described in the above, since the detection of the number of wavelengths as described above is not performed, the automatic level control can be performed at a high control speed of millisecond order. As a result, it becomes possible to suppress the variation of signal output level of the optical amplifier to be less, even in the case where the span loss is varied at a high-speed.
To be specific, the comparison will be made between the automatic level control using the above described wavelength numbers information and the automatic level control by the span loss monitoring. For example, in the case where the span loss is varied at a high-speed as a result that the transmission path fiber is swung, a signal input waveform to the optical amplifier arranged on the downstream of the transmission path fiber is significantly varied during a period of time from the variation of span loss occurs at the time T0 until the variation of span loss is ended at the time T1, as shown in the upper stage of FIG. 7 for example. In such a case, if the automatic level control using the wavelength numbers information is applied to this optical amplifier, since it is necessary to set the speed of the automatic level control to be sufficiently lower than the detecting time of the number of wavelengths, a signal output waveform from the optical amplifier is in the form in which the variation of the signal input level is represented just as it is, as shown in the middle stage of FIG. 7 for example.
On the other hand, if the automatic level control by the span loss monitoring is applied to the optical amplifier, since the high-speed automatic level control of millisecond order can be performed, the signal output waveform from the optical amplifier is in the form in which the variation of the signal input level is suppressed, as shown in the lower stage of FIG. 7 for example. Accordingly, in the automatic level control by the span loss monitoring, it becomes possible to hold an optimum signal level from the standpoint of signal reception sensitivity.
However, in the optical amplifier to which the automatic level control by the span loss monitoring as described above is applied, there is a problem in that a measurement error of the span loss is directly linked to a setting error of the signal output level. Further, as shown in FIG. 8 for example, in the case where optical amplifying units 201 to 203 to each of which the automatic level control by the span loss monitoring is applied, are connected in multi-stages via transmission path fibers 211 to 213, there is a problem in that a measurement error of the span loss in each optical amplifying unit is accumulated. To be specific, for example, when actual span losses in the transmission path fibers 211, 212 and 213 among first to third repeating sections are 22 dB, 25 dB and 23 dB, if the span losses calculated in the respective optical amplifying units 201, 202 and 203 are 21.5 dB, 24.5 dB and 22.5 dB, an error of −0.5 dB occurs in a target value of the signal output level, which is set in each of the optical amplifying units 201, 202 and 203. Therefore, in the signal light after sequentially passed through the three-staged optical amplifying units 201, 202 and 203, the level reduction of 1.5 dB occurs.